The media rights group has identified them as Olena Maksymenko of Ukrainian weekly Ukrainsky Tizhden and freelance photographer Oles Kromplyas.
It says Maksymenko disappeared with two activists in the Auto-Maidan movement, which supports the new government in Kiev, after they were stopped at a checkpoint in Perekop.
Reporters Without Borders said Kromplyas was taken into custody by pro-Russian forces shortly afterward at the same checkpoint.
Christophe Deloire, the secretary general of Reporters Without Borders, expressed concern Monday over a "steady escalation in violations of journalists' rights in Crimea, which is turning into a lawless region controlled by armed bands whose anonymity reinforces the impunity."
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